The present invention relates to an aircraft fuel system comprising a fuel storage facility for storing fuel and ullage. The present invention also relates to an aircraft with such a fuel system and a method of operating an aircraft.
Aircraft fuel is stored in one or more aircraft tanks. Air that contains fuel vapour, such as the air above the fuel in the fuel tanks, is known as ullage. When an aircraft is refuelled, the pressure in the fuel tanks is high and, in order to prevent a large difference in pressure between the tank and the atmosphere (to prevent damage to the wing), ullage is expelled into the atmosphere. Similarly, upon aircraft ascent, ullage is expelled to the atmosphere. This causes fuel vapour to be emitted, which may be harmful to the environment.
Upon aircraft descent, especially after fuel has been depleted, in order for the pressure in the fuel tanks to be increased to reduce the pressure difference between the fuel tanks and the atmosphere, air is injected back into the fuel tanks.
In addition, the air that has been injected back in is often treated to reduce its oxygen content so as to maintain low levels of oxygen in the aircraft tanks and to reduce the flammability risk of the fuel tanks. The inerting system often is required to produce a large amount of inert (low oxygen level) air upon descent and this results in a weight, drag and power consumption penalty for the aircraft. Also, when air is injected back into the tanks, water is often ingested too. This is because the humid outside air is drawn into the tanks and the water then condenses on the fuel and the cold surfaces within the tank. This leads to regular water drain maintenance being required and may result in fuel system equipment failures due to, for example, water ingress into valves and pressure switches, or switches seizing up or seals malfunctioning due to freezing.
An example of an inerting system is shown in EP 2439141. This discloses an aircraft fuel system where ullage is taken from the fuel tank, using engine bleed air, processed through a main catalytic unit, to reduce its oxygen content, and returned to the fuel tank. The engine bleed air is used to control the speed of ullage flow through the main catalytic unit, based on the temperature in the unit. In other words, the ullage flow is caused to bypass the main catalytic unit and return to the fuel tank (to reduce the amount of reaction occurring) if the main catalytic unit is getting too hot. The inerting system is known as a GOBIGGS (Green On Board Inert Gas Generation System).
The present invention seeks to mitigate the above-mentioned problems. Alternatively or additionally, the present invention seeks to provide an improved aircraft fuel system.